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HCPL-4504-000E Datasheet, PDF (19/19 Pages) AVAGO TECHNOLOGIES LIMITED – High CMR, High Speed Optocouplers
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The LED signal to turn on Q2 should be delayed enough
so that an optocoupler with the very fastest turn-on
propagation delay (tPHLmin) will never turn on before an
optocoupler with the very slowest turn-off propagation
delay (tPLHmax) turns off. To ensure this, the turn-on of the
optocoupler should be delayed by an amount no less
than (tPLHmax - tPHLmin), which also happens to be the max-
imum data sheet value for the propagation delay differ-
ence specification, (tPLH- tPHL). The HCPL-4504/0454/J454
and HCNW4504 specify a maximum (tPLH- tPHL) of 1.3 μs
over an operating temperature range of 0-70°C.
Although (tPLH-tPHL)max tells the designer how much delay
is needed to prevent shoot-through current, it is insuffi-
cient to tell the designer how much dead time a design
will have. Assuming that the optocoupler turn-on delay
is exactly equal to (tPLH - tPHL)max, the minimum dead time
is zero (i.e., there is zero time between the turnoff of the
very slowest optocoupler and the turn-on of the very
fastest optocoupler).
Calculating the maximum dead time is slightly more
complicated. Assuming that the LED turn-on delay is still
exactly equal to (tPLH - tPHL)max, it can be seen in Figure 17
that the maximum dead time is the sum of the maximum
difference in turn-on delay plus the maximum difference
in turnoff delay,
[(tPLHmax-tPLHmin)+(tPHLmax-tPHLmin)].
This expression can be rearranged to obtain
[(tPLHmax-tPHLmin)-(tPHLmin-tPHLmax)],
and further rearranged to obtain
[(tPLH-tPHL)max-(tPLH-tPHL)min],
which is the maximum minus the minimum data sheet
values of (tPLH-tPHL). The difference between the maxi-
mum and minimum values depends directly on the total
spread in propagation delays and sets the limit on how
good the worst-case dead time can be for a given design.
Therefore, optocouplers with tight propagation delay
specifications (and not just shorter delays or lower pulse-
width distortion) can achieve short dead times in power
inverters. The HCPL-4504/0454/J454 and HCNW4504
specify a minimum (tPLH- tPHL) of -0.7 μs over an operat-
ing temperature range of 0-70°C, resulting in a maximum
dead time of 2.0 μs when the LED turn-on delay is equal
to (tPLH-tPHL)max, or 1.3 μs.
It is important to maintain accurate LED turn-on delays
because delays shorter than (tPLH - tPHL)max may allow
shoot-through currents, while longer delays will increase
the worst-case dead time.
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Data subject to change. Copyright © 2005-2009 Avago Technologies. All rights reserved. Obsoletes AV01-0552EN
AV02-0867EN - January 4, 2010